Field of the Invention
One disclosed aspect of the embodiments relates to a fixing device that fixes a toner image formed on a recording material by heating, and an image-forming apparatus using the fixing device such as an electrophotographic printer or a copying machine.
Description of the Related Art
A fixing device of a film-heated type, subsequently mentioned, has been known as a fixing device mounted on a conventional image-forming apparatus (Japanese Patent Laid-Open No. 2000-131974).
The fixing device includes a cylindrical fixing film that is heated by a heater and a pressure roller that forms a nip portion together with the heater with the fixing film interposed therebetween. In the fixing device, a recording material on which a toner image is formed is heated at the nip portion while being conveyed, and the toner image is fixed on the recording material.
In the fixing device, a failure in an image called “end tailing” may occur. The tailing means a phenomenon in which water vapor emitted from the inside of the recording material causes a toner image to be scattered in the direction opposite the direction in which the recording material is conveyed in the case where the toner image is fixed by heating on the recording material that is hygroscopic.
In Japanese Patent Laid-Open No. 2000-131974, a method of applying bias having the same polarity as a toner to the fixing film to form an electric field in the direction in which the toner is attracted to the recording material is disclosed as a countermeasure against the tailing. The electric field causes the toner to be pressed against the recording material. Consequently, the scatter of the toner image due to water vapor is suppressed.
In addition to the tailing failure, a failure in an image called an “offset” that occurs in fixing devices has been known. The offset means a phenomenon that occurs such that a surface of the pressure roller is gradually charged up while the recording material is conveyed at the nip portion, and consequently, an unfixed toner on the recording material is ripped off.
In Japanese Patent Laid-Open No. 2010-128474, a method of applying bias having a polarity opposite to the polarity of a toner to the fixing film or the pressure roller during a period in which the recording material does not pass through the nip portion to prevent the pressure roller from being charged up is disclosed as a countermeasure against the offset.
In recent years, however, it is difficult for the problems of the “offset” and “end tailing” to be solved at the same time because the print speed of image-forming apparatuses is increased, and the period in which the recording material does not pass through the nip portion is short. The reason will be described.
The “offset” is caused by the force of an electric field that causes the toner to be attracted to the fixing film and that is generated when a recording material P is fed and the pressure roller is charged up so as to have the same polarity as the tonner. The image-forming apparatus, which is difficult to ensure a sufficient time interval between sheets, cannot ensure a sufficient time for applying the bias having a polarity opposite to the polarity of the toner in the time interval between sheets.
Consequently, a sufficient electrical charge cannot be applied to the pressure roller. This reduces the effect of preventing the charging-up, and the “offset” may occur.
In order to apply a sufficient electrical charge having a polarity opposite to the polarity of the toner to the pressure roller in a short time interval between sheets, the time for applying the bias having a polarity opposite to the polarity of the toner to the fixing film in the time interval between sheets can be increased. This, however, shortens a switching time between performing switching such that the bias having the same polarity as the toner is applied when a succeeding recording material P passes through a nip portion N and the succeeding recording material P passing through a fixing nip. Consequently, the “end tailing” may occur.